[mlir][ODS] Add `OptionalTypesMatchWith` and remove a custom assemblyFormat (#68876)

This is just a slight specialization of `TypesMatchWith` that returns
success if an optional parameter is missing.

There may be other places this could help e.g.:

https://github.com/llvm/llvm-project/blob/eb21049b4b904b072679ece60e73c6b0dc0d1ebf/mlir/include/mlir/Dialect/X86Vector/X86Vector.td#L58-L59
...but I'm leaving those to avoid some churn.

This constraint will be handy for us in some later patches, it's a
formalization of a short circuiting trick with the `comparator` of the
`TypesMatchWith` constraint (devised for #69195).

```
TypesMatchWith<
  "padding type matches element type of result (if present)",
  "result", "padding",
  "::llvm::cast<VectorType>($_self).getElementType()",
  // This returns true if no padding is present, or it's present with a type that matches the element type of `result`.
  "!getPadding() || std::equal_to<>()">
```

This is a little non-obvious, so after this patch you can instead do:
```
OptionalTypesMatchWith<
  "padding type matches element type of result (if present)",
  "result", "padding",
  "::llvm::cast<VectorType>($_self).getElementType()">
```